Images of the Sacred Heart meticulously recount key details of the crucifixion. The wounded heart itself, the crown of thorns, and the cross itself all appear. Some depictions even include the lance that pierced the side of Christ penetrating His heart.
But there’s one detail that seems out of place. There was no fire at the crucifixion, yet the Sacred Heart is often shown with flames. Why?
A burnt offering. Recall that Christ’s sacrifice on the cross was mean to recapitulate and supersede all the sacrifices of the Old Testament. What was a common feature of these sacrifices? Fire. Think of the fire that devoured the sacrifices offered by Elijah and the fire that Abraham would have set had an angel not intervened (see 1 Kings 18 and Genesis 22). In ancient Israel, a burnt offering was the supreme form of sacrifice, it symbolized a total commitment to God—particularly the death of the victim animal and the all-consuming nature of the fire. (Key sources here, here, and here.) The burning Sacred Heart reminds us that this sacrifice too was incorporated into Christ’s supreme offering of Himself on the cross.
Symbol of divinity. Of course, fire is also a familiar Old Testament symbol of God. We encounter God’s fiery presence at Sinai and in the account of Ezekiel (see Ezekiel 1). This symbolism carries over into the Old Testament, where the Holy Spirit descends upon the heads of the apostles as tongues of fire. Perhaps it’s especially fitting that the Sacred Heart is burning given that from it poured water and blood, symbols of the sacraments of baptism and the Eucharistic wine, both the work of the Holy Spirit.
Symbol of the divine Incarnate. The fire burns, but the Sacred Heart is not consumed. Does this sound familiar? It recalls Moses’ first encounter with God, in a bush that burned but was not consumed. This foreshadowed the Incarnation, in which God assumed human nature, without his divinity extinguishing the humanity that had been assumed: Christ was fully man and fully God. It is fitting that at this climactic moment of the Incarnation that its deepest reality is reaffirmed in such an acute way.
Jesus’ passion for us. In the context of the gospels, the Passion refers to the suffering of Christ. But, in our society, we usually use the word passion to refer to something or someone that drives our enthusiasm, interest, desires, and commitments. Is this meaning still valid for the Sacred Heart? I think so. There is evidence in the gospels that a burning heart signified intense emotions. One clear example of this is the two disciples who encountered Christ on the road to Emmaus and afterwards remarked that their hearts had been burning. (See Luke 24; my source for this interpretation is here.) So yes, the flames on the Sacred Heart are a true reminder of God’s burning love for us.
Light of the World. Fire does two things. First, it consumes that which it burns. Second, it gives off light. This second aspect is certainly relevant to the symbolism of the Sacred Heart, given that Christ is the true light of the world. Remember that during the crucifixion, darkness descended upon the land (see Mark 15:33). In the darkest hour, the Sacred Heart burned bright with hope.